From the discovery of superconductivity in 1911 to the recent past, essentially all known superconducting materials were elemental metals (e.g., Hg, the first known superconductor) or metal alloys and intermetallic compounds (e.g., Nb.sub.3 Ge, probably the material with the highest transition temperature T.sub.c known prior to 1986).
Recently, superconductivity was discovered in a new class of materials, namely, metal oxides. See, for instance, B. Batlogg, Physica, Vol. 126, 275 (1984), which reviews superconductivity in barium bismuth lead oxide, and J. G. Bednorz and K. A. Muller, Zeitschr. f. Physik B--Condensed Matter, Vol. 64, 189 (1986), which reports superconductivity in lanthanum barium copper oxide.
The latter report stimulated worldwide research activity, which very quickly resulted in further significant progress. The progress has resulted, inter alia, to date in the discovery that compositions in the Y--Ba--Cu--O system can have superconductive transition temperatures T.sub.c above 77 K, the boiling temperature of liquid N.sub.2 (see, for instance, M. K. Wu et al, Physical Review Letters, Vol. 58, Mar. 2, 1987, page 908; and P. H. Hor et al, ibid, page 911). Furthermore, it has resulted in the identification of the material phase that is responsible for the observed high temperature superconductivity, and in the discovery of compositions and processing techniques that result in the formation of bulk samples of material that can be substantially single phase material and can have T.sub.c above 90 K (see, for instance, R. J. Cava et al, Physical Review Letters, Vol. 58(16), pp. 1676-1679).
The excitement in the scientific and technical community that was created by the recent advances in superconductivity is at least in part due to the potentially immense technological impact of the availability of materials that are superconducting at temperatures that do not require refrigeration with expensive liquid He. Liquid nitrogen is generally considered to be perhaps one of the most advantageous cryogenic refrigerant, and attainment of superconductivity at liquid nitrogen temperature was a long-sought goal which until very recently appeared almost unreachable.
Although this goal has now been attained, there still exist barriers that have to be overcome before the new high T.sub.c superconductive compounds can be utilized in many technological applications. In particular, the oxide superconductive bodies are relatively brittle and mechanically weak, and have relatively low electrical and thermal conductivities in the normal state. Thus, it would be desirable to be able to produce superconductive oxide bodies having improved mechanical, electrical and/or thermal properties. This application discloses such bodies, and a technique for producing them.
For a general overview of some potential applications of superconductors see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor Applications: SQUIDS and MACHINES, Plenum Press 1977; and S. Foner and B. B. Schwartz, editors, Superconductor Material Science, Metallurgy, Fabrications, and Applications, Plenum Press 1981. Among the applications are power transmission lines, rotating machinery, and superconductive magnets for e.g., fusion generators, MHD generators, particle accelerators, levitated vehicles, magnetic separation, and energy storage, as well as junction devices and detectors. It is expected that many of the above and other applications of superconductivity would materially benefit if high T.sub.c superconductive material could be used instead of the previously considered relatively low T.sub.c materials. Since in many of the applications the current through the superconductive body will be interacting with large magnetic fields, the body will experience large Lorentz forces. In these and other cases, superconductive bodies of improved mechanical properties, including toughness and strength, could be advantageously used.
Improved mechanical properties are of course desirable in general, since in many cases the superconductive body will have to withstand stresses due to handling, installing, and the like, and always will have to withstand stresses due to thermal contraction and expansion. Furthermore, it is typically desirable for the normal-state electrical and thermal conductivities of superconductive body such as wire or ribbon to be relatively high, such that, in the case of local loss of superconductivity, heat generation is minimized and the generated heat can be rapidly conducted away.
The prior art knows several techniques for producing superconductive oxide bodies. See, for instance, R. J. Cava et al, Physical Review Letters, Vol. 58, page 1676 (1987), U.S. patent application Ser. No. 025,913, filed Mar. 16, 1987 for E. M. Gyorgy et al, titled "Apparatus Comprising a Ceramic Superconductive Body, and Method for Producing Such a Body", and U.S. patent application Ser. No. 034,117, filed Apr. 1, 1987 for S. Jin et al, titled "Apparatus and Systems Comprising a Clad Superconductive Oxide Body, and Method for Producing Such a Body", both incorporated herein by reference. The former patent application discloses a technique for making superconductive oxide bodies having at least one small dimension (e.g., sheetings, tapes, thin rods, and the like). The technique comprises mixing the oxide powder with a binder, and heating the "green" body under conditions such that the resulting ceramic body is superconductive.
The latter patent application discloses a technique for making an elongate normal metal-clad oxide superconductive body, e.g., a wire or tape, which comprises forming an intermediate body that comprises a cladding material surrounding a quantity of oxide powder, forming elongate body by reducing the cross section of the intermediate body (e.g., by rolling or by drawing through dies), and by heat treating the elongate body under conditions such that sintering of the oxide powder occurs, and such that the sintered oxide body is superconductive.
In general, the techniques insure that the oxide body is not "poisoned" during the heat treatment, that the oxide has the appropriate crystal structure, and that the oxygen content of the heat treated oxide is in the range that is associated with superconductivity in the oxide.